Circuit Failure Mechanisms
This is a summary of the different circuit failure modes and situations provided in ELEC3106 lectures by Torsten Lehmann. Electro-Static Discharge (ESD) Electro-Static Discharge occurs when a person or object has collected a large static charge, often by rubbing up against an insulator, and then touches a conductor. Whilst the energy discharged is relatively minor, the instantaneous voltage and current can be quite high, and hence can cause undesired results in a circuit. The most common form of failure due to ESD is reverse breakdown, where the ESD voltage is high enough to cause reverse current in a semiconductor. Often this completely destroys the semiconductor (leaving it permanently open or short circuit) rendering the circuit useless. Electromigration Due to the non-ideal nature of conductors, electrons regularly bump into positively charged particles when a current is flowing. This causes tiny movements in the structure of a conductor, and over time causes noticeable migration of positive particles within the conductor. Eventually this will cause an open circuit, rendering the component useless. It is only really an issue on very small conductors, mainly those on integrated circuits. Insert picture here. Electromigration is a problem with both AC and DC currents, however high DC currents will cause the greatest problems. The effect is accelerated when there is a defect or fault in the wire, as more collisions and therefore more migration occurs there. A typical lifespan for an IC before electromigration causes an open circuit is about 10 years. Dendritic Growth and Dielectric Breakdown In an ideal world, there would be no ions in dielectrics between conductors. In reality, there will always be a small number of ions between conductors, and therefore in the presence of a large magnetic field some particles can corrode from one conductor and get deposited on the other. This eventually causes dendrites, which are conductive strands extending from one conductor to the other. Over time this can cause short circuiting between the conductors. Insert picture here. The presence of ions can also lead to dielectric breakdown, normally in locations where the electric field is not uniform (i.e. around defects, dendrites, etc). This will cause a short circuit, which in some cases causes permanent destruction of the component. Photo-Lithographic Defects Most PCBs and ICs are fabricated using photo-lithographic processes, whereby the pattern you desire is projected on to the substrate and a chemical reaction takes place to create the component. Depending on the size of the component and the size of the screen being used to project, dust particles can become an issue. If the projected dust particle is comparable to the size of conductors or dielectrics, then the addition or removal of conductive material due to dust can cause problems. There are two cases, complete failure and near failure. In a complete failure, the addition or subtraction of conductive material completely open or short circuits a section of the component, and causes failure. This is normally easily detected by simple testing at the time of manufacture, and the components that fail can be discarded. In a near failure, it creates near open or near short circuits, which accelerate electromigration and dendritic growth. This is often undetectable at the time of manufacture, and usually leads to early circuit failure. Junction Spiking In chemistry we learnt that if there are different concentrations of things on different sides of a liquid, they will begin to disperse throughout the liquid. The same is true with p-type and n-type semiconductor junctions! At standard temperatures it isn't always a big deal, but for example silicon is soluble in aluminium. This can lead to spikes of one type inside another type, and can eventually burst through to the substrate, effectively disabling the component. Insert picture here. Parameter Shift All of the other phenomena above will obviously change the characteristics of the device over time, resulting in a change of parameters. Once the component's parameters stray far enough outside the values for which the circuit was designed, the circuit can fail. Stress and Thermal Cycling As the materials used to construct components and circuit boards are different, it seems logical that they will expand at different rates when heated or cooled. Solder has very little play in it, so normally the only way to allow for the growth and contraction of the circuit board is to mount components on flexible legs. When this is not done, and components are mounted simply on the surface of the circuit board, thermal cycling can cause cracking of the solder, and result in short circuits. Contact Wear Exposed surfaces will tend to develop an oxide coating, which forms a thin layer of poorly conducting material. Normally, when a contact is made it will crack the oxide layer and make a good electrical connection. Over time however the oxide layer builds up and the connection gets worse and worse until eventually the connection fails. Apparently this can be delayed by using noble metals for connectors, although I thought only gases could be noble... Corrosion This is fairly obvious. Circuits on a PCB can be corroded by corrosive chemicals. Makes sense. Apparently these chemicals can get inside ICs too. Cosmic Radiation This is just getting ridiculous. Apparently as storage media gets smaller, the potential effects of radiation causing electron-hole pairs in certain places becomes an issue. This is mainly through bit-flips, causing memory corruption. It can be prevented to a degree with shielding, however a better way to avoid it is to make sure that data corruption doesn't result in failure, i.e. using redundancy, checksums, etc. Category:ELEC3106 Category:Circuits